1. Field of the Invention
This invention is directed to methods of forming a glass sheet (glass ribbon) and, in particular, to methods of forming a glass sheet using controlled cooling between the location where the sheet is formed (e.g., the root of an isopipe) and the location where individual substrates are separated from the sheet (e.g., the location where the ribbon is scored as the initial step of the separation process).
2. Technical Background
Glass display panels in the form of liquid crystal displays (LCDs) are being used in an increasing variety of applications—from hand-held personal data assistants (PDAs) to computer monitors to television displays. These applications require glass sheets which have pristine, defect-free surfaces. LCDs are comprised of at least several thin sheets of glass which are sealed together to form an envelope. It is highly desirable that the glass sheets which comprise these displays do not deform when cut, thereby maintaining the proper registration, or alignment, between the elements. Residual stress which may be frozen into the glass, if relieved by cutting the glass into smaller portions, may result in deformation of the glass, and a loss of proper registration.
Typically, LCDs are of the amorphous silicon (α-Si) thin film transistor (TFT) or polycrystalline-silicon (ρ-Si or poly-Si) TFT type. Poly-Si has a much higher drive current and electron mobility, thereby decreasing the response time of the pixels. Further, it is possible, using ρ-Si processing, to build the display drive circuitry directly on the glass substrate. By contrast, α-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques.
The evolution from α-Si to ρ-Si has presented a major challenge to the use of a glass substrate. Poly-Si coatings require much higher processing temperatures than do α-Si, in the range of 600-700° C. Thus, the glass substrate must be thermally stable at such temperatures. Thermal stability (i.e. thermal compaction or shrinkage) is dependent upon both the inherent viscous nature of a particular glass composition (as indicated by its strain point) and the thermal history of the glass sheet as determined by the manufacturing process. High temperature processing, such as required by poly-Si TFTs, may require long heat treatment times for the glass substrate to ensure low compaction, e.g., 5 hours at 600° C.
One method of producing glass for optical displays is by an overflow downdraw process (also known as a fusion downdraw process). This process produces pristine surface quality compared to other processes referred to as the float and slot techniques in the literature. U.S. Pat. Nos. 3,338,696 and 3,682,609 (Dockerty), which are incorporated in their entirety herein by reference, disclose a fusion downdraw process which includes flowing a molten glass over the edges, or weirs, of a forming wedge, commonly referred to as an isopipe. See also U.S. Patent Publications Nos. 2005/0268657 and 2005/0268658, the contents of which are also incorporated herein in their entireties by reference. The molten glass flows over converging forming surfaces of the isopipe, and the separate flows reunite at the apex, or root, where the two converging forming surfaces meet, to form a glass ribbon, or sheet. Thus, the glass which has been in contact with the forming surfaces is located in the inner portion of the glass sheet, and the exterior surfaces of the glass sheet are contact-free. The sheet as it evolves decreases in thickness under the forces of gravity and pulling equipment. In particular, pulling rolls are placed downstream of the isopipe root and capture edge portions of the ribbon to adjust the rate at which the ribbon leaves the isopipe, and thus help determine the thickness of the finished sheet. The pulling equipment is located sufficiently downstream so that the glass has cooled and become rigid enough to be pulled. The contacted edge portions are later removed from the finished glass sheet. As the glass ribbon descends from the root of the isopipe past the pulling rolls, it cools to form a solid, elastic glass ribbon, which may then be cut to form smaller sheets of glass.
The construction of a fusion downdraw line requires a substantial capital investment. Because substrates produced by such a line are typically employed in the manufacture of consumer products (see above), there exists a continual pressure to reduce costs. Such cost reductions can be achieved by, among other things, increasing a line's output and/or by decreasing the costs; e.g., capital costs, involved in constructing the line. As discussed below, various aspects of the present invention can be used to implement either or both of these cost reduction approaches, i.e., these aspects of the invention can be used to increase draw speed and thus the output of a line and/or can be used to decrease the overall length of a line, e.g., the vertical height between the root of the isopipe where the glass sheet is formed and the bottom-of-the-draw where substrates are separated from sheet. (As known in the art, after separation from the glass sheet, substrates undergo further processing, e.g., removal of the bead portions at the sides of the substrate, subdivision into smaller pieces, edge grinding, etc., before being used in the manufacture of, for example, liquid crystal displays. The word “substrate” is used herein and in the art to refer to both the individual panes separated from the glass ribbon prior to any further processing and the ultimate substrate used by LCD manufacturers, it being evident from the context which meaning is applicable.)
Thermal instability of glass substrates used in the production of liquid crystal displays has been a longstanding problem in the art. To address this problem, glass manufacturers often heat treat glass substrates prior to shipping them to customers so that the sheets do not shrink or shrink very little when used in the customers' processes. Such heat treatments are known as “pre-shrinking” or “pre-compacting.” The heat treatments involve further handling of the substrates thus increasing the chances of damage to the surfaces of the substrates, as well as increasing overall manufacturing costs.
Quantitatively, compaction is the change in length per unit length exhibited by a glass substrate as a result of subtle changes in glass structure produced by thermal cycling (i.e., compaction is strain resulting from the glass' thermal history). Compaction can be determined physically by placing two marks on a glass substrate and measuring the initial distance between the marks. The substrate is then subjected to a heat treatment cycle and returned to room temperature. The distance between the marks is then re-measured. Compaction in parts-per-million (ppm) is then given by:compaction=106·(distance before−distance after)/(distance before).
Various heat treatment cycles can be used to simulate the heating and cooling that a substrate will experience during, for example, the manufacture of a liquid crystal display. Examples of suitable heat treatment cycles that can be used to determine the expected compaction of glass substrates are set forth below (see Table 4).
In addition to physically testing glass substrates, compaction can also be predicted using computer models for the stress relaxation of glass materials when subjected to prescribed temperatures for prescribed periods of time. Examples of such modeling appear in Buehl, W. M., and Ryszytiwslkyj, W. P., “Thermal Compaction Modeling of Corning Code 7059 Fusion Drawn Glass”, SID International Symposium, Digest of Technical Papers, SID 22, 667-670 (1991). See also Narayanaswami, O. S., “Stress and structural relaxation in tempering glass”, J. Amer. Ceramic Soc., 61 (3-4) 146-152 (1978). The models are semi-empirical in that a fit is made to measured strains resulting from a variety of thermal cycles applied to a particular type of glass, and then the fit is used to predict compaction for the thermal history of interest, e.g., thermal histories of the type shown in Table 4. The compaction data set forth below was obtained using the semi-empirical modeling approach as opposed to physical testing.
Because compaction is an important end customer specification, historically, as flow increases have been made, i.e., throughput increased, the fusion process has been scaled linearly to allow sufficient time at temperature to maintain the same compaction of the finished substrates as existed before the flow increase. Although this approach does work, it has the serious drawback that it requires longer distances between the root of the isopipe and the location where substrates are separated from the glass sheet. These longer distances take up additional real estate and capital. Indeed, due to the physical constraints of existing facilities, this approach to dealing with compaction can limit the maximum flow available to a given glass forming installation. Increasing the flow beyond these historical and physical constraints would provide a significant and important cost advantage.
Another limitation of the fusion draw process as currently practiced relates to the material properties of the glass to be processed. It is well known that when a glass composition initially in the molten state is exposed to a lower temperature for a significant amount of time, the development of crystal phases will initiate. The temperature and viscosity where these crystal phases start to develop is known as the liquidus temperature and liquidus viscosity, respectively.
As known and currently practiced, when using the fusion draw process it is necessary to maintain the viscosity of the glass at the location where it leaves the isopipe at a viscosity greater than about 100,000 poises, more typically greater than about 130,000 poise. If the glass has a viscosity lower than about 100,000 poise, the quality of the sheet degrades, e.g. in terms of maintaining the sheet flatness and controlling the thickness of the sheet across its width, and glass sheet thus produced is no longer suitable for display applications.
According to current practice, if a glass composition which has a liquidus viscosity of less than about 100,000 poises is processed under conditions such that the dimensional quality of the glass sheet would be adequate, devitrification may develop on the isopipe and lead to crystalline particulates in the glass sheets. This is not acceptable for display glass applications.